Most university pages will have such information.
But they are not official sources of information (i.e. where did they get their information from?).
The UKBA website is the official source for visa and immigration information - and there's no guarantee that what it says on any university website is entirely accurate unless it is also stated on the UKBA website.
I believe it will be the case that if you apply for the visa beforehand it will be multiple-entry as the UKBA website says that almost all visit visa Entry Clearances was issued as multiple entry (mentioned here:
http://www.ukba.homeoffice.gov.uk/policyandlaw/guidance/ecg/vat/vat1/, but not on the student visitor guidance:
http://www.ukba.homeoffice.gov.uk/policyandlaw/guidance/ecg/vat/vat9/).
What I cannot find on the UKBA website is whether or not the stamp given at the border if you don't apply in advance is multiple entry (I don't think it is, but I can't find any official confirmation of this).
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If you will be applying for a student visitor visa, you must return to the US to apply for it (as others have said), as you can only apply from the country you have legal residence in.
There are a number of things you must show in order to be able to get the visa:
- You must have been accepted on a course of study for less than 6 months and have evidence of this from the academic institution (must be on official headed paper from the university, detailing the course title, the duration of the course and the cost of the course).
- You must show that you will be leaving the UK after your studies and have no inention of living here (for example, you will need evidence of a home and job or studies in the US to return to)
- You must show that you do not intend to marry while in the UK, that you will not work (paid or unpaid) and that you will not use the NHS.
- You have enough money to pay for your return ticket home (if you don't apply for the visa beforehand and just get it on entry to the UK, you will need to have a return ticket with you).